In middle and long distance optical communication systems, high-capacity communications are growing using high-speed and multiple-wavelength features. In the present trunk-line optical communication systems, wavelength multiplexing telecommunications are used. In the wavelength multiplexing telecommunications, the wavelength channel spacing is defined. Thus, when the wavelength channel spacing within the bandwidth of an optical fiber amplifier is 50 GHz, about 100 channels are available.
It is assumed that the channel spacing is Δf[Hz], and the transmission rate is B[bit/s]. The frequency efficiency in this case is represented as B/Δf [bit/s/Hz]. When Δf=50 GHz and the transmission rate of each channel is 100 Gbit/s, the frequency efficiency is 2 bit/s/Hz.
Because the optical fiber amplifier bandwidth is limited, it is necessary to enhance the frequency efficiency. However, when the signal bit rate is merely increased in order to enhance the frequency efficiency, the problem of crosstalk between channels is raised. Thus, as the next generation optical communication system, multilevel optical modulation and orthogonal frequency-division multiplexing (OFDM) are under review.
The multilevel optical modulation is a modulation scheme that converts the amplitude and phase of light into multiple values, not only binary values of either 0 or 1. This increases the amount of information without increasing the frequency band. Further, in the optical OFDM, an OFDM signal is generated from an electrical signal and optically modulated and then multiplexed with optical sub-carriers being orthogonal to one another. As a result, it is possible to solve the problem of crosstalk and enhance the frequency efficiency.
The optical signal that is transmitted by multilevel modulation/multiplexing principally involving electrical signal processing is demodulated into the electrical signal at the receiving end. An analog-to-digital (A-D) converter is required in the subsequent stage of a photodetector (PD)
of the optical demodulation circuit. An A-D converter using an electrical circuit is generally used today.
On the other hand, an A-D converter that derives an analog value of an optical signal directly as a digital signal is advantageous in its speed. Therefore, many proposals for the A-D converter that derives an analog value of an optical signal directly as a digital signal have been made. For example, in Patent Literature 1, the amount of light is represented by a specified ratio by dividing an optical signal at a specified dividing ratio. Then, an optical analog value of the input optical signal is detected depending on whether each divided optical signal reaches a threshold.
Further, in Patent Literature 2, a feedback system through a nonlinear optical element is constructed for an input optical signal, which is an analog signal, in an optical A/D conversion means. First output light, which is a digital signal, is thereby sequentially obtained from the optical A/D conversion means.
In Patent Literature 3, an optical encoding circuit that processes encoding and quantization by an optical signal is disclosed. As the optical encoding circuit, a plurality of optical encoders including optical nonlinear elements in which input/output characteristics for light intensity have different periodicities are used. A pulse string of signal light having a first wavelength is optically encoded according to control light, which is a pulse string of an optical analog signal having a second wavelength different from and close to the first wavelength and being an optical sampling signal, and a plurality of pulse strings of the encoded signal light are output from each of the optical encoders. Then, an optical quantization circuit performs optical threshold processing of a pulse string of carrier light having a third wavelength different from and close to the first wavelength according to the plurality of pulse strings of the encoded signal light using a plurality of optical threshold processors including optical nonlinear elements in which input/output characteristics for light intensity have periodicities and thereby makes quantization, and then outputs it as an optical digital signal.
In Patent Literature 4, an optical A/D conversion device including a plurality of interferometric optical modulators and photovoltaic elements formed on the same substrate is disclosed. The optical A/D conversion device has a feature that the output voltage of the photovoltaic element is applied to the interferometric optical modulator. In this example, intensity signal light received by a PD is converted from optical to electrical. Therefore the rate of the entire circuit is determined by the rate of the electrical signal after conversion.
Further, a phase difference, not limited to an intensity signal, may be used as a signal for light. An example using this technique is disclosed also in some of the above Patent Literatures. Further, there is an example that creates a phase difference from intensity, using light as it is, without performing photoelectric conversion for use. In Patent Literature 5, a logic holding/logic inversion signal generator that converts an optical signal that is on or off into a phase difference signal is disclosed.
In Patent Literature 6, a device that removes control light by a filter using a phase difference modulation signal is disclosed.